A Study of a PH-Sensitive Polymer for Novel Conformance Control Applications




Choi, Suk Kyoon

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Use of pH-sensitive crosslinked poly(acrylic acid) polymer has been proposed as a novel in-depth conformance control system for improved oil recovery. The polymer forms a soft and flexible microgel with a molecular network structure in aqueous solution. When dispersed, it becomes acidic and exists in an unswelled and coiled state. High swelling occurs at neutral pH, accompanied by an increase in viscosity of several orders of magnitude. The objective for developing this system is to propagate microgels under acidic condition deep into the high permeability zone which has been swept by the injected fluid. Once placed, microgels swell as pH-buffering reactions with mineral components occur. The swelled microgels provide resistance to subsequent fluid flow, leading to an improved waterflood profile. New laboratory studies were conducted to develop this novel polymer-gel system, and to investigate its technical feasibility. The development in this thesis is divided into three broad tasks, which include: 1) viscosity measurements of pH-triggered polymer-gel systems at different pH, shear rate, polymer concentration, and ionic strength; 2) geochemical characterization in terms of spatial and temporal change in pH when a dilute hydrochloric acid preflush is injected into a sandstone core; and 3) characterization of polymer microgel transport in porous media in terms of its molecular interaction parameters. Laboratory viscosity measurements indicated that the apparent viscosity of a crosslinked poly(acrylic acid) is strongly dependent on pH, justifying its use as a conformance control agent. The ionic strength, one of the reservoir variables, was found to be inversely proportional to swelling ratio, i.e, the apparent viscosity. Since high ionic strength interrupts the swelling of microgels, it should be kept as low as the salinity of reservoir brine. Apparent viscosity was also observed to be proportional to crosslink density. Therefore, a high crosslink density is preferred over a low crosslink density, because a lower concentration is required to achieve a given viscosity. As a preflush, a 0.1M hydrochloric acid is inefficient because of its weak reactivity with silicate minerals. Core flood experiments showed that the alumino-silicate minerals in Berea sandstone core must be completely dissolved before a low pH environment can be established, which is necessary for good propagation of microgels. Thus, use of more concentrated hydrochloric acid or a highly reactive chemical should be considered for use in a preflush stage. In experiments of microgel transport in pure-silica sandpacks, a poly(acrylic acid) polymer with high crosslink density was found to form more rigid microgel particles than a polymer with low crosslink density. The rigid particles have weaker attractive adsorption characteristics, leading to less retention. Addition of anionic surfactant helps to effectively reduce the inter-particle attraction between microgels by binding hydro-phobically to particles. Retention decreased by approximately 40% over a surfactant free flooding case, but, could not be completely eliminated. It is concluded in this study that an aqueous solution of Carbopol® 934 which has the highest crosslink density available, with the addition of an anionic surfactant, can propagate deep into a porous medium with one order of magnitude permeability reduction. Such a polymer-surfactant combination can be applied in a relatively high permeability zone for improved conformance control.


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